I find that kind of critical mass a little bit surprising, even with a beryllium reflector and a homogenous reactor.
I'm not sure what library he used for it; some of these libraries are less reliable than others. The reactor is designed to produce neutrons for boron therapy; my guess is that there are better, simpler sources, in particular the old reliable Cf-252.
The author, Yigal Ronan, has written elsewhere on the isolation of Am-242m: Detailed Design of 242mAm Breeding in Pressurized Water Reactors. This paper refers on the difficult task of obtaining relatively pure Am-242m. He approaches it by arranging strong neutron absorbers around the Am-241 target.
This I think, is counter-productive, and doesn't exploit the high multiplicity which Americium isotopes provide.
Am-242 represents 4 captured neutrons. This is best achieved in breeder situations; otherwise the formation of Americium represents lost neutrons. Since the multiplicity of americium fission across the neutron spectrum is reportedly higher than 3, the use of americium fuel helps recover these neutrons for other uses.
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